Electrically powered vehicles such as electric vehicles and hybrid vehicles have attracted a lot of attention as environmentally friendly vehicles. These vehicles incorporate a motor for generating a driving force for running, and a rechargeable power storage device for storing power supplied to the motor. A hybrid vehicle is a vehicle incorporating a motor as well as an internal combustion engine as a driving source, a vehicle incorporating a power storage device as well as a fuel cell as a direct current power supply for driving the vehicle, or the like.
As with an electric vehicle, a hybrid vehicle having a vehicle-mounted power storage device that can be charged from a power supply outside of the vehicle is known. For example, a so-called “plug-in hybrid vehicle” is known in which a power storage device can be charged from a power supply at an ordinary household by connecting a power supply outlet provided at the house to a charging inlet provided on the vehicle by a charging cable.
Wireless power transmission without using a power cord or a power transmission cable has been receiving attention in recent years as a power transmission method. Power transmission using electromagnetic induction, power transmission using a microwave, and power transmission by resonance are three dominant techniques for wireless power transmission.
The resonance is a wireless power transmission technique for causing a pair of resonators (e.g., a pair of coils) to resonate with each other in an electromagnetic field (near field) to transmit power through the electromagnetic field, and can transmit a large amount of power of several kW across a relatively long distance (e.g., several meters).
Japanese Patent Laying-Open No. 2010-141976 (PTL 1) discloses a wireless power transmission apparatus for transmitting power in a contactless manner to a vehicle by resonance. This wireless power transmission apparatus includes an alternating current power source, a primary coil connected to the alternating current power source, a primary-side resonant coil, a secondary-side resonant coil, and a secondary coil connected to a load (secondary battery), and further includes an impedance varying circuit arranged between the alternating current power source and the primary coil. The primary coil, primary-side resonant coil, secondary-side resonant coil, secondary coil and load form a resonant system. An impedance of the impedance varying circuit is adjusted in such a manner that an input impedance of the resonant system at a resonant frequency matches with an impedance of the alternating current power source side excluding the primary coil.
According to this wireless power transmission apparatus, power can be efficiently supplied from the alternating current power source to the load without changing the frequency of the alternating current power source even if the distance between the resonant coils or the load receiving the power changes (see PTL 1).